1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling cell selection and related communication device.
2. Description of the Prior Art
Wideband Code Division Multiple Access (WCDMA) is adopted in third generation (3G) mobile communication systems as a wireless access technology, for providing high spectrum efficiency, universal coverage, and multimedia transmission with high quality and high data rate. Besides, various quality of service (QoS) requirements can be met at the same time, two-way transmissions with flexibility and diversity are provided, signal quality is improved, and a probability that communication is interrupted is reduced. Furthermore, a universal mobile telecommunications system (UMTS), one of the 3G mobile communication systems, provides a dual/multi mode function with which a user equipment (UE) can perform an inter-system handover, such as a handover between 2G and the 3G mobile communication systems, to enhance transparency between different mobile communication systems for the UE.
A large geographic area is divided into a large number of cells in a wireless communication system, such as the 3G mobile communication system, for reusing wireless resources (e.g. times, frequencies and/or codes) in different cells, to utilize the wireless resources efficiently. Further, neighboring cells are usually grouped into a location area, to reduce an amount of location update procedures performed by the UE. Thus, an amount of control information transmitted/received by the UE can be reduced, and a probability that a mobile terminated (MT) call for the UE is missed can also be reduced.
For example, please refer to FIG. 1, which is schematic diagram of a wireless communication system 10 according to the prior art. The wireless communication system 10 includes location areas LA1, LA2 and LA3. Further, the location area LA1 includes cells CL_1, CL_2 and CL_3, the location area LA2 includes cells CL_4 and CL_5, and the location area LA3 includes a cell CL_6. When a UE UE_P is in a coverage of the cell CL_1, a network of the wireless communication system 10 not only transmits information of the UE UE_P to the cell CL_1, but also transmits the information of the UE UE_P to other cells of the location area LA1, e.g., the cells CL_2 and CL_3. Thus, when the UE UE_P moves from the coverage of the cell CL_1 to a coverage of the cell CL_2, the cell CL_2 can identify the UE UE_P, and the UE UE_P does not need to perform a location update procedure. Thus, an amount of control information transmitted/received by the UE UE_P can be reduced, and a probability that a MT call for the UE UE_P is missed can also be reduced. Oppositely, when the UE UE_P moves from the coverage of the cell CL_1 to a coverage of the cell CL_4, the cell CL_4 does not have the information of the UE UE_P, and can not identify the UE UE_P. Thus, the UE UE_P needs to perform the location update procedure, for providing the information of the UE UE_P to the cell CL_4. In this situation, the UE UE_P can not perform a data transmission or a voice call during the location update procedure (e.g., several seconds). Further, the UE UE_P may move across multiple location areas. For example, the UE UE_P first moves from the cell CL_1 to the cell CL_5, and then moves from the cell CL_5 to the cell CL_6. In this situation, the UE UE_P may not perform the data transmission or the voice call over 10 seconds, and inconvenience is caused to the UE UE_P.
On the other hand, a UE receives signals transmitted by neighboring cells and signals transmitted by a cell with which the UE currently communicates. Then, the UE measures qualities of the signals received, and feeds back the signal qualities to a network. According to the signal qualities fed back by the UE, the network indicates to the UE which cells should be added in active sets of the UE, and which cells should be removed from the active sets. In other words, the UE maintain the active sets according to the signal qualities of the cells, wherein signal qualities of cells in the active sets are better than signal qualities of other cells. The UE can access a service via the cells in the active sets, and can perform a handover with some or all of the cells in the active sets when moving. Further, the UE can select a cell from the cells in the active sets for camping on the cell, when the UE transfers to an idle mode.
In general, after the UE finishes a data transmission or a voice call, the UE transfers from a connected mode to the idle mode. The UE should determine a cell for camping in the idle mode, and the UE determines a cell with a best signal quality for camping according to the prior art. However, the cell with the best signal quality and a cell with which the UE finishes the data transmission or the voice call are not necessarily in a same location area. If the cell with the best signal quality and the cell with which the UE finishes the data transmission or the voice call are not in the same location area, the UE needs to perform the location update procedure before transferring to the idle mode. Thus, the UE can not perform the data transmission or the voice call again for seconds.
For example, please refer to FIG. 2, which is schematic diagram of signal qualities of cells in active sets of the UE UE_P in FIG. 1 according to the prior art. In FIG. 2, the active sets of the UE UE_P include the cells CL_1 and the CL_4 which are in the location areas LA_1 and LA_2, respectively. At the beginning, the UE UE_P camps in the idle mode, and camps on the cell CL_1. After the UE UE_P is triggered by a data transmission or a voice call and transfers from the idle mode to the connected mode, the cell CL_4 starts to serve the UE UE_P. The UE UE_P can perform the data transmission or the voice call via the cells CL_1 and CL_4 simultaneously, and quality of the data transmission or the voice call can be improved. After the data transmission or the voice call is finished, the UE UE_P transfers back to the idle mode again. Since signal quality of the cell CL_4 becomes better than signal quality of the cell CL_1 due to movement of the UE UE_P or variation of environment, the UE UE_P selects the cell CL_4 for camping on the cell CL_4 in the idle mode. However, the cells CL_1 and CL_4 are not in the same location area, and the UE needs to perform the location update procedure before transferring to the idle mode. Thus, the UE can not perform the data transmission or the voice call again for seconds. Therefore, how to determine a suitable cell for camping in the idle mode is an important topic to be discussed and addressed.